The objective of the proposed research is to gain a better understanding of the processes of potassium secretion and reabsorption by the nephron and how these processes are altered by disturbances in acid-base balance, by changes in blood and tissue electrolyte content, by diuretic agents and by other factors. The bull frog kidney will be used because it has a portal circulation that perfuses tubular tissue but is not filtered. Both the arterial and portal circulations to the kidney will be isolated and artificially perfused. By perfusing the portal circulation with K42 and with the use of clearance techniques and various kinetic methods, measurements of secretion and reabsorption rates and some of the unidirectional flux rates between cells and tubular fluid and between cells and the circulation will be made. The magnitude of the cellular pools of K will also be measured.